NEW YORK (AP) — Novak Djokovic and Andy Murray both had their moments of concern before winning in the second round of the U.S. Open on Friday, taking strides toward a seemingly destined semifinal clash.

The top-seeded Djokovic faced two early set points against him and needed a tiebreaker to win the first set before overwhelming Benjamin Becker of Germany 7-6 (7-2), 6-2, 6-2.

Leonardo Mayer, ranked 81st, ran Murray all over the court and temporarily threatened an upset when he took the third set but the defending champion won the last five games for a 7-5, 6-1, 3-6, 6-1 victory.

It was another former champion, though, who pulled off the most impressive result of the day, as Lleyton Hewitt turned back the clock to in a stirring 6-4, 5-7, 3-6, 7-6 (2), 6-1 victory over 2009 winner Juan Martin del Potro.

Murray too looked like he might fall victim to a persistent opponent when Mayer snatched the third set, but the Scot was able to steady and quickly get through the fourth for victory.

"He's a big hitter of the ball. I had to defend a lot," Murray said in an on-court interview. "He played some really, really good tennis. It made for an entertaining match."

Becker had a chance to serve for the first set against Djokovic at 5-4. But he wasted the first set point with a forehand into the net and the second when a backhand sailed wide.

"Becker is a quality opponent and he should have won the first set," Djokovic said. "I was fortunate enough to come back and win the first set, and after that, I was much more comfortable on the court."

Djokovic was playing in Arthur Ashe Stadium, where the wind often swirls to players' frustration.

"You need to have this adjustment, footwork, steps, in order to get on the ball," Djokovic said. "I didn't have that in the first set. I was still trying to find my way from baseline, and the other side he was serving well. ... After I played a good tiebreak, everything kind of settled. I started to serve better, started to step into the court, which is important."

Murray was in Louis Armstrong Stadium, a place that had bedeviled him in the past. Last year, he was pushed to four tough sets in the third round and quarterfinals there.

Another Grand Slam winner, Li Na, also showed some championship form on Ashe. This time, her opponent, Laura Robson, looked very much like a teenager.

The fifth-seeded Li avenged her third-round upset loss to the young Brit at last year's U.S. Open, winning in straight sets at the same stage at Flushing Meadows.

Li came from a break down in the second set for a 6-2, 7-5 victory, crediting a pep talk from coach Carlos Rodriguez.

"After the talk I was feeling much, much better," she said. "Because before I never try to share the feeling with the team."

Stephens' next opponent could be defending champ Serena Williams, who faces Yaroslava Shvedova in the nightcap at Ashe. It would be a rematch of their Australian Open quarterfinal, won by the young American.

Juan Martin Del Potro (ARG) during his match against Lleyton Hewitt (AUS) on day five of the 2013 US Open in Ashe Stadium at the Billie Jean King National Tennis Center. (Photo: Robert Deutsch, USA TODAY Sports)